The present invention relates to an image forming apparatus which is structured to ensure stable amount of light when organic EL elements are used as light emitting elements in a line head and to reduce the deterioration of the elements.
In conventional image forming apparatus in which a latent image is written on an image carrier, it is common practice to employ an LED (light emitting diode) array as writing means. Line head in which light elements such as LEDs are aligned in plural lines has been developed. For example, Japanese Patent No. 2534364 discloses such a line head in which EL elements are used to form arrays. As the application of driving pulse to the EL elements is finished, the afterglow is reduced. Accordingly, as the driving pulse is applied after non-emission for a long time, the time required to reach a predetermined light intensity becomes longer and, in addition, the amount of emitted light becomes smaller. For this, in Japanese Patent No. 2534364 as the conventional example, an auxiliary pulse is applied at least once in one main scanning to light all of the EL elements. By applying the auxiliary pulse as mentioned above, the time required to reach the predetermined light intensity becomes short even after non-emission for a long time. The auxiliary pulse is set to have such intensity not to expose a photoreceptor and to produce afterglow. That is, the light emission is started in the state that afterglow exists.
Disclosed in Japanese Patent Publication No. H8-32468 is an examples of a line head in which light emitting elements using inorganic EL elements are aligned in plural arrays. For driving the line head using the inorganic EL elements, driving pulses are always applied from electrodes on both surfaces and the synchronous of these driving pulses is controlled to retain the potential of the synchronized pulse not to exceed a threshold value, thereby controlling the emission of light. However, according to this control method, direct current bias is applied to inorganic EL elements even during non-printing. As a pulse of which positive and negative are asymmetrical is applied, the deterioration inside a thin film of each inorganic EL element proceeds due to its characteristics, thus lowering the light output of the inorganic EL element. That is, as a direct current bias is applied to the inorganic EL element during the non-printing, the light output is lowered. For this, in Japanese Patent Publication No. H8-32468, a pulse of which positive and negative are symmetrical is applied during non-printing so as to prevent the deterioration of inorganic EL elements.
Further, organic compound as a component of the organic EL element has a characteristic of being susceptible to water. Therefore, Japanese Patent Unexamined Publication No. 2000-127488 describes a technique to solve this problem. That is, the temperature of the organic EL elements is detected and the remaining heat is controlled to retain the temperature of the organic EL elements when standby.
In the conventional example disclosed in the aforementioned Japanese Patent No. 2534364, the auxiliary pulse having such intensity as to produce afterglow is applied to the organic EL elements. This example has a problem that the deterioration of the EL elements is accelerated because the organic EL elements are actually lighted. In addition, in case of using organic EL elements as light emitting elements, the amount of emitted light is increased as the temperature increases due to application of voltage. That is, there is a problem that the amount of emitted light varies according to the variation in temperature of the organic EL elements. Because the deterioration of the organic EL element is accelerated by the light emission, there is also a problem that differences in degree of deterioration among the elements lead to variation in light emission.
In the conventional example disclosed in Japanese Patent Publication No. H8-32468, inorganic EL elements which deteriorate by application of direct current voltage are used and there is no disclosure about technology for correcting such variation in light emission among organic EL elements.
In the conventional example disclosed in Japanese Patent Unexamined Publication No. 2000-127488, there is no disclosure about the intensity of voltage to be applied to the organic EL elements in order to control the temperature. Application of voltage exceeding the light emitting voltage shortens the lives of the organic EL elements. There is a problem that structure of the line head is complex because a temperature detecting means is provided and the control circuit is complex because a temperature control circuit is added.